1. Field of the Invention
The present invention relates to a transfer and fixing device used in electronic photography, electrostatic recording, and electrostatic printing, and to an image forming apparatus comprising such a transfer and fixing device.
2. Description of the Related Art
Commonly known for many years are image forming apparatuses configured such that an image is formed on an image carrier (photosensitive member) using a developing device, that image is transferred to an intermediate transfer member using a primary transfer device, the image on the intermediate transfer member is further transferred to a transfer material using a secondary transfer device, and the image on the applicable transfer material is fixed using a fixing device.
In addition, image forming apparatuses configured to conduct the above processing steps in a sequential manner have been in general use, and there have also been proposals for image forming apparatuses comprising a so-called transfer and fixing device that can conduct the transfer processing step and the fixing processing step simultaneously, such as Japanese Patent Application Publication No. H10-63121 (called Prior Art 1 hereinafter) and Japanese Patent Application Publication No. 2004-145260 (called Prior Art 2 hereinafter).
Indicated in the aforementioned Prior Art 1 is an image forming apparatus configured to conduct secondary transfer and fixing from the intermediate transfer member onto transfer material, and indicated in the aforementioned Prior Art 2 is an image forming apparatus configured to conduct tertiary transfer and fixing from the intermediate transfer member onto transfer material after secondary transfer and fixing onto a transfer and fixing member from the intermediate transfer member.
Toner (electrically charged powder with a main component of resin) is generally used in image forming technology. The transfer processing step in this image formation is a step that is prone to invite a drop in image quality.
Paper is mainly used as the recording medium on which the image is formed, but the thickness may vary from ordinary paper to thick paper. Moreover, there is also a variety of surface characteristics from high quality to coarse. In particularly, when utilizing coarse surface paper, micro-gaps may be formed in which the surface of the paper cannot be followed, leading to the disadvantage that these micro-gap parts generate abnormal electrical discharge causing the image to become thin overall without transferring normally.
In contrast to this, in image forming apparatuses comprising a transfer and fixing function that simultaneously conducts the transfer and fixing processing steps have the advantage that this drop in image quality is not likely to occur even when using coarse surface recording medium (paper). This is because simultaneously applying pressure and heat in the transfer processing step softens and fuses the toner to become a viscoelastic block-shaped agglomerate, and transfer can be conducted in positions equivalent to the micro-gaps parts of the surface of the paper.
Because of the foregoing, it may be said that an image forming apparatus provided with a transfer and fixing device is the optimum for achieving high image quality.
Further, with a transfer and fixing device, there is no traveling with powder riding on the recording medium, and therefore there is the advantage that a transport guide can be set up with a configuration that does not narrowly limit the paper passage direction until immediately in front of the transfer and fixing unit, which has the advantage of being able to transport thick to thin paper or other paper types corresponding to a variety of conditions. There is a high degree of freedom in handling paper types in this way, which can effectively reduce the percentage of paper jams generated.
In common electronic photography, because powder rides on the recording medium to just in front of the transfer and fixing unit, and because the transport guide can only guide by setting up a gap such that this powder is not rubbed off, the recording medium becomes unstable within this gap, and many paper jams are generated.
In this regard, in order to sufficiently heighten the heat efficiency in the transfer and fixing processing step, it is necessary to heighten the temperature on the surface that fuses the recording medium (paper) and the toner, specifically, the interface between paper and toner. In the past, a system was used in which thoroughly heated and softened toner was pressurized onto the paper. Nonetheless, because not only the toner, but also the transfer and fixing member was heated in order to obtain sufficient effect in this system, if, for example, thick transfer and fixing material of 300 μm is used, and specifically if a four-drum tandem imaging system or the like is adopted and the perimeter is long, then it may not be possible to guarantee sufficient thermal efficiency. Further, cooling must be included in the latter processing step, resulting in a configuration that must both heat and cool the same member, and therefore this is an extremely disadvantageous configuration from the perspective of energy efficiency.
Technologies, for example, Japanese Patent Application Publication No. 2005-37879, have been proposed that focus on the aforementioned problem and selectively heat the recording medium (paper) itself immediately prior to contact with the toner.
Nonetheless, in this system there is still the problem of producing temperature fluctuations, specifically, there is the defect that so-called scumming is prone to occur when printing multiple pages. Moreover, because heating to the rear surface, as is conducted in conventional transfer and fixing, does not contribute to the fixing of the recording medium and wastes energy, it is preferable to increase the temperature of only the transfer surface of the recording medium prior to transfer, and to be able to prevent a temperature drop, but this kind of technology has not yet been proposed.
Means that provide a plate-shaped heating member or a high temperature rotational member and heat the recording medium to be transported (may be called “paper” hereinafter) has been considered as a method to increase the temperature of just the transfer surface of the recording medium immediately prior to transfer. Nonetheless, in either system there is the problem that the paper is separated from contact with the heating member and the paper surface temperature decreases in the interval up to arriving at the nip where the toner image is transferred, and specifically, there is the problem that the temperature drop becomes marked if the paper is thick.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent No. 3042414 and US 2008/0199229.